What Climate Overshoot Means Now That 1.5°C Is Gone
A new UN report concedes the Paris Agreement's 1.5°C limit will be crossed within years. What happens next is called "overshoot" — and it will shape climate policy for the rest of the century.
1.4 degrees Celsius. That is roughly how much hotter the planet has already gotten compared with pre-industrial times, and this week the United Nations stopped pretending the next few tenths of a degree are avoidable.
A new UN Environment Programme report, released Wednesday in Nairobi, concedes that the 1.5°C limit set by the 2015 Paris Agreement will be crossed "in the next few years." That is not a modeling worst case. It is the UN's own central expectation, laid out in a document called, bluntly, Limiting Overshoot. "Humanity is speeding past the limit to avoid climate catastrophe," UN Secretary-General António Guterres said in response to the report.
The word doing the real work here is overshoot, and it is worth understanding on its own terms, because it is about to become the organizing concept of climate policy for the rest of the century.
What "overshoot" actually describes
Overshoot is not a euphemism for failure, though it is partly that too. Technically, it describes a temperature trajectory that exceeds an agreed limit, holds above it for some stretch of time, and then, if emissions fall far enough, comes back down below that limit later. Global temperatures, on this reading, don't have to cross 1.5°C and stay there forever. They can spike, plateau, and eventually recede, the way a fever breaks.
The report's most-cited number is the shape of that spike. In the most optimistic scenario, where governments pursue genuinely ambitious cuts, warming peaks around 1.8°C (3.2°F) above pre-industrial levels by mid-century before declining. Under the policies countries have actually adopted so far, the UN's modeling instead points toward roughly 2.6°C (4.7°F) by 2100. Every five years that meaningful emissions cuts are delayed adds at least another tenth of a degree to that peak, according to Imperial College London climate scientist Joeri Rogelj, one of the report's co-authors.
None of this makes 1.5°C meaningless. "The 1.5 limit is not some magical climate cliff," University of Victoria climate scientist Andrew Weaver, who was not involved in the study, told the Associated Press. "Every tenth of a degree we avoid still means less damage."
Living above the line
What changes practically is the UN's own guidance to governments. Instead of telling policymakers to prevent 1.5°C, UNEP Executive Director Inger Andersen said officials must now "prepare for and navigate a future" in which the world crosses that line and tries to correct course afterward. Richard Betts, a report co-author and climate scientist at the University of Exeter, was more direct still: "It's a reality check. We have to live with this warmer world. But there's still a lot we can do to limit warming." He estimated the overshoot period itself could stretch across "many, many decades" even under a reasonably successful response.
The report ties two kinds of action together and treats neither as optional. The first is immediate, steep cuts to greenhouse gas emissions, methane included, on a path toward net zero. The second, longer-term piece is net-negative emissions: actively pulling carbon back out of the atmosphere once net zero is reached, through reforestation and newer carbon dioxide removal technology. The report is candid about the limits of the low-tech version of that plan. Tree planting alone, even at ambitious scale, could shave at most a tenth of a degree off global temperatures, which is why the authors say costlier, unproven removal methods will be needed too. Carbon accounting for those removals is already a live fight in its own right — the systems that retire carbon credits rather than let them expire will only face more scrutiny as overshoot policy leans harder on removal math.
Every additional fraction of a degree during the overshoot period raises the odds of crossing what the report calls tipping points: destabilization of the West Antarctic and Greenland ice sheets, dieback of the Amazon rainforest, and disruption of the Atlantic Meridional Overturning Circulation, the ocean current system that helps keep Western Europe's climate mild. Some of these shifts could happen abruptly; others would accumulate over years. The report also points to this summer's flooding in Nepal, triggered in part by a dislodged Himalayan glacier, as a preview of the kind of disaster that gets more likely, not less, the longer temperatures stay elevated.
Not everyone thinks the UN has fully reckoned with what it is proposing. Bill Hare, CEO of Climate Analytics, said the report "does a good job of describing the hole we've dug ourselves into" but "a poor job of showing us that there's a way out." That tension, honest about the scale of the problem and thinner on the mechanics of the recovery, runs through most of the reaction to the report so far.
The politics of getting there will play out at COP31, co-hosted by Türkiye and Australia with Pacific island nations, where the pathway laid out this week is expected to shape the negotiating agenda. In the meantime, Stanford climate scientist Rob Jackson, who was not part of the report, put the stakes in the plainest terms anyone has managed so far. "We were unable to stop the runaway train of global emissions to avoid 1.5°C of warming," he said. "Now overshoot scenarios require us to stop the train quickly and to back it up. Is it our best hope? Absolutely. Is it likely? Absolutely not."
